The broad goal of this experimental program is to understand how neurons in striate cortex encode visual information and how this information is processed by neuronal microcircuits. We will ultimately express this knowledge in a model that embodies cortical architecture, the function of individual cells, the coding of visual information and the interactions between cells. We believe that the next major progress in understanding the processing of visual information, and brain function in general, will result from the study of the interaction of cells within cortical networks. We propose a program that coordinates investigation of network influences on response properties, observation of the interaction between cells as these influences are realized, delineation of anatomical pathways specific to these interactions and expression of these results in a comprehensive, quantitative model. The program has three related parts. Natural vision involves complex stimuli, so we will assess the influence of network interactions resulting from stimuli with two or more components on the structure of cortical receptive fields in the cat. We will try to localize the origin of stimulus-based reorganization of the receptive fields within the cortical layer structure by testing while pharmacologically blocking the activity of other specific regions. In these experiments, by injection of biocytin at the blocking site we will identify the morphology of the cells involved as well as trace the anatomical pathways from the blocking site to the recording site. We will also test for the presence of GABA at these sites. The second goal of the program is to study how functional connectivity within the cortical network depends on the nature of the stimulation. We will simultaneously record from coupled neurons using a method of spike discrimination to determine the relative effectiveness of single spikes and spike bursting in intracortical communication. These experiments will also help us to understand the extent to which changes in cell-to-cell communication are responsible for receptive field filter characteristics and adaptive effects that are spatially- or contrast-dependent. The third goal is to express our findings in the form of a quantitative, realistic model that will include both pyramidal and stellate cells, layered organization, complex membrane mechanisms and network interactions dictated by our previous results and the results of others. The development of the model and experimental planning will be interactive, with experiments suggesting new aspects of the model and the model suggesting new types of experiments.